Yongfang Li

High performance all-small-molecule solar cells: engineering the nanomorphology via processing additives

The use of small volumes of solvent additives (SAs) or little amounts of non-volatile additives is a processing approach that has been implemented in many high/record performing bulk heterojunction (BHJ) organic solar cells (OSCs). Here, the effects of six SA systems and a molecular additive di-2-thienyl-2,1,3-benzothiadiazole (DTBT) were studied with respect to the photovoltaic parameters of solution-processed all small molecule solar cells (all-SMSCs) based on the BDTT-S-TR:NIDCS-MO system. An effective strategy with binary additives has been employed in this all-SM system, where a small amount, 0.75 vol% 1,8-diiodooctane (DIO) and 2 wt% DTBT were added to the casting solution. This efficient SA approach yielded the highest power conversion efficiency (PCE) of 5.33%. The relevant additives facilitate phase separation in the nm domains and improve bulk transport as evidenced by photoluminescence (PL), atomic force microscopy (AFM), X-ray diffraction (XRD) and space charge limited current (SCLC) measurements.

Towards a full understanding of regioisomer effects of indene-C60 bisadduct acceptors in bulk heterojunction polymer solar cells

Indene-C60 bisadduct (IC60BA), which can offer a significantly higher open-circuit voltage (Voc) than monoadducts, has become the research focus as electron acceptor materials in polymer solar cells (PSCs) in recent years. However, despite its popularity, IC60BA have always been applied in PSCs as mixture of several regioisomers and the nature of this mixture has never been fully investigated and understood. Herein, for the first time, 12 major regioisomers of IC60BA were isolated and a full investigation was carried out with respect to their structure, abundance, solubility and their corresponding photovoltaic performance. The results show that the PSCs based on these regioisomeric structures present very diverse PCE and their photovoltaic performance was dramatically affected not only by the relative indene positions but also by the steric orientation of the two indene groups. Electrochemical studies further revealed that the effect of energetic disorder inside the IC60BA regioisomers on their photovoltaic performance is insignificant when applied in PSCs. However, the steric structures and solubility of the regioisomers were found to have significant impact on the morphology and bulk properties of the active layer of PSCs, which give rise to very different PCE of devices based on IC60BA regioisomers with different structures.

Conjugated Oligothiophene Derivatives Based on Bithiophene with Unsaturated Bonds as Building Blocks for Solution-Processed Bulk Heterojunction Organic Solar Cells

A new building block ATVTA that uses stiff carbon-carbon triple bonds (A) on 1,2-di(2-thienyl)-ethene (TVT) has been developed. Oligothiophene derivatives S-01 with a TVT unit, S-02 with a 5,5-diethynyl-2,2-dithienyl (AT2) unit and S-03 with ATVTA were synthesized to compare their effects in a systematic study. Due to the better π-conjugation extension of the TVT unit, S-01 exhibits the most red-shifted absorption profile among them, whereas S-02 possesses the deepest HOMO level. While the HOMO level of S-03 is down-shifted by 0.02u2005eV relative to that of S-01, the alkyne linkages can effectively down-shift the HOMO level. By replacing the terminal units of S-03 with stronger electron acceptors, S-04 and S-05 exhibited broader absorption profiles and lower HOMO levels than those of S-03. Organic solar cells based on these molecules were fabricated and an S-03:PC60 BM (1:1, w/w) based device afforded the highest Voc value of 0.96u2005V and a power conversion efficiency (PCE) of 2.19u2009%.